Sema: Relax enum parameter pack restriction for CodingKeys

lib/Sema/TypeCheckDeclPrimary.cpp

@@ -3226,6 +3226,7 @@ public:
 
     // Temporary restriction until we figure out pattern matching and
     // enum case construction with packs.
+    if (!ED->isSynthesized()) {
       if (auto genericSig = ED->getGenericSignature()) {
         for (auto paramTy : genericSig.getGenericParams()) {
           if (paramTy->isParameterPack()) {
@@ -3234,6 +3235,7 @@ public:
           }
         }
       }
+    }
 
     // FIXME: Remove this once we clean up the mess involving raw values.
     (void) ED->getInterfaceType();

test/decl/protocol/special/coding/codable_parameter_pack.swift

@@ -0,0 +1,7 @@
+// RUN: %target-typecheck-verify-swift -target %target-swift-5.9-abi-triple
+
+// We should accept this:
+
+public struct HasPack<each T>: Codable {
+  var x: String?
+}